1. Field of the Invention
This invention relates to a heater construction particularly suitable for use in hot isostatic pressing apparatus, and more specifically to a heater of a compact construction which can ensure uniform heating in vertical direction in a high temperature environment involving vigorous free convections and which is easy to assemble.
2. Description of the Prior Art
Recently, ceramic materials such as silicon carbide, silicon nitride and so-called Sialon have attracted attention for application to the heat-resistant high-strength component parts like turbine blades of hot gas turbine engines, nozzles and heat exchangers, while boron carbide is regarded as an excellent friction resistant material. In order to solve the problems which lie in the way to application of these ceramic materials as engineering ceramics, there have thus far been developed high density sintering methods for realizing the inherent properties of these materials and methods for enhancing reliability by reducing irregularities. The hot isostatic pressing (hereinafter referred to simply as "HIP" for brevity) which is employed in the processes of fabrication of cemented carbide parts for sintering a work item at a high temperature and in an isostatically pressed state by using an inert gas as a pressurizing medium is regarded as the most prospective process. However, in order to apply the HIP process to the engineering ceramics for high densification sintering to thereby obtain products of high reliability, it is necessary to employ a temperature above 1700.degree. C. for silicon nitride and Sialon, a temperature above 1850.degree. C. for silicon carbide and a temperature above 2000.degree. C. for boron carbide even in a high pressure gas atmosphere of 1000 kgf/cm.sup.2. The hot isostatic pressing apparatus (hereinafter referred to simply as "HIP apparatus" for brevity) which can maintain such a high temperature stably along with uniform heating is still in the stage of development.
The heater, including the above-mentioned HIP apparatus, which is essential to the generation of a high temperature above 1700.degree. C. employs in most cases a heating element of high melting point metal such as molybdenum, tantalum and tungsten or graphite. However, this type of heater which uses a high melting point metal invariably suffers from the problems of creep deformation which occurs during use over a long time period and the coarsening of crystal grains due to repeated thermal cycles, causing embrittled fracture at low temperatures, in addition to an economical problem in that it is extremely costly and unsuitable for a large apparatus. Although graphite can solve these problems, it is barely usable in a large apparatus due to the difficulty of reducing the sectional area of the heating element and the necessity for cooling the joint portions to the metal electrodes during use because of its extremely high heat conductivity.
These are not exceptions even in the HIP apparatus. With the recent developments in the research of the graphite type heater, it has become possible to construct an electric heater which is capable of generating high temperature above 2000.degree. C., further increasing the opportunities for practical applications of the HIP apparatus.
The conventional HIP apparatus is usually provided in its furnace chamber with a cylindrical heater which is, as illustrated in FIG. 1, constituted by a cylindrical heating element 2' for heat generation, a metal electrode 13 fixedly mounted on a stationary plate 14 through an insulator 5, and a number cylindrical posts 6' serving as electrode rods and secured to the heating element 2' by threaded engagement of screw portions 7' at the upper ends of the posts 6' with tapped holes 16 formed in the lower end of the heating element 2' for connecting the metal electrode 13 to the heating element 2'. The heater construction with a heating element 2' connected to a cylindrical posts 6' in this manner permits facilitated centering when assembling the respective parts owing to the small Young's modulus of the flexible graphite heating element 2', which instead has a drawback in that it easily breaks due to fragility of the material and thus requires careful handling. Further, when part of the heating element 2' is broken or damaged, it becomes necessary to remove it along with the cylindrical posts 6' at the time of replacement with a fresh heating element, resulting in low working efficiency.
In order to eliminate the foregoing problems or drawbacks, there has been proposed a heater construction as shown in FIG. 2, in which the heating element 2' is provided with through holes 5' in a flange portion at its lower end and fixedly secured to the cylindrical posts 6' by inserting screw members 7' of the cylindrical posts 6' through the holes 5' and tightening nuts 9 on the screw members 7'. This heater construction can eliminate the drawback of the heater of FIG. 1 but still has an inherent problem that the through holes 5' have to be located on the outer side of the outer periphery of the heating element 2' and at a space therefrom by increasing the radial dimension of the heater as indicated by letter A to provide an ample space around the nuts 9 to permit the same to be easily turned with a tool. It follows that the heater has a larger outside diameter as compared with a heater of the same inside diameter, necessitating providing a high pressure container of a larger inside diameter which is disadvantageous from the standpoint of compactness of the HIP apparatus.
The just-mentioned problem can be solved by reducing the width of the flange to provide the through holes 7' substantially in the same radial positions as the heat generating portions (hatched portions) of the heating element 2' as shown particularly in FIG. 3. Similarly to the heater construction of FIG. 2, it is still necessary to provide a free space around each nut 9 for threading the same onto the screw member 7' by providing notches or void portions (.alpha.) in the heating element at positions corresponding to the respective cylindrical posts 6'. The provision of such void portions in the heating element is however undesirable because of the impairment of uniform heating function of the heater.